The small intestine is the longest section of the gastrointestinal tract, and due to its anatomical location, structure and physiological characteristics, traditional gastroscopy can only examine a small portion of the small intestinal mucosa with direct vision, while other imaging examinations such as small intestine air-barium imaging, nuclear scan, CT and magnetic resonance imaging (MRI) have limited diagnostic value for small intestinal diseases. Therefore, for a long time, the small intestine has been a blind area for gastrointestinal examination, and small intestinal diseases are difficult to be diagnosed correctly due to the limitation of examination means. In recent years, with the invention and application of capsule endoscopy and balloon small intestine microscopy, it has become possible to examine the whole small intestine, and the diagnosis level of small intestine diseases has made a qualitative leap. Advantages of capsule endoscopy: Compared with traditional endoscopy, capsule endoscopy has the advantages of easy operation, non-invasive, non-cross-infection, easy to tolerate, and no need for sedation. Moreover, the clear color images taken provide physicians and patients with images of the whole gastrointestinal tract and a high diagnostic rate for small intestinal lesions, making it an ideal method for small intestinal disease screening and a promising application in high-end medical examinations. For unexplained small bowel bleeding, capsule endoscopy is the safest and most effective diagnostic method, with a diagnostic rate of 38% to 93%. Also, capsule endoscopy has a certain detection rate for esophageal, gastric and colonic diseases. Korman et al. conducted a preliminary study of GI motility and transmission time using capsule endoscopy and concluded that it has some value in the study of GI dynamics. With the development of technology, the pixels of pictures taken by capsule endoscopy have improved substantially and are approaching high-definition standards; the recorders worn with the patient are designed to be increasingly lightweight to improve patient comfort; and portable real-time monitoring provides the possibility for physicians and patients to observe the workings of the capsule at any time. Limitations of capsule endoscopy: Currently, capsule endoscopy has some drawbacks, such as the maximum field of view of the endoscope is only 140°, which is not wide enough and has a short visual field, making it difficult to observe larger or more distant lesions and the full circumference of the dilated intestinal wall. Further, capsule endoscopy takes images in the intestinal tract as a random act and is not more selective and targeted, which does not allow for focused observation of suspicious lesions. Capsule endoscopy mainly relies on the peristaltic waves of the gastrointestinal tract to move forward. If the moving speed is too slow, the examination will take a long time, and even the whole small intestine cannot be examined, thus decreasing the detection rate of small intestinal diseases; if the moving speed is too fast, the presence of intestinal lesions may not be detected or clarified. Moreover, the capsule moves with the peristaltic wave and its direction cannot be controlled artificially, which also makes it easy to miss detection. In addition, capsule endoscopy currently only visualizes objects and does not allow for endoscopic biopsy and related treatment. Determinants of successful capsule endoscopy: First, the presence of intestinal contents can interfere with the visualization of the intestinal mucosa. Usually, patients fast 12h before the examination, can drink water 2h after the start of the examination, and can eat a simple meal 4h later. Second, the gastric emptying time and the speed of small bowel peristalsis can affect the speed of the capsule running forward, resulting in the inability to complete a full small bowel examination. Studies have shown that the incidence of failure to complete a full small bowel examination is approximately 25%. Bowel cleansing medications (e.g., polyethylene glycol solution, oral sodium phosphate) and the antifoam agents dimethicone oil or pro-gastrointestinal agents (e.g., metoclopramide, domperidone, tegaserod, erythromycin) may improve small bowel cleansing and may increase the success rate of total small bowel exams. However, there is still no consensus on the optimal type of medication, dose, and timing of administration. In addition, it has been suggested that in patients with unexplained gastrointestinal bleeding, the fluid applied during bowel preparation may wash away blood from the intestine and thus affect the localization of the bleeding lesion. Therefore, the suitability of intestinal preparation such as catheterization in patients with unexplained gastrointestinal bleeding still needs further study. Contraindications and complications of capsule endoscopy: Severe dysmotility, GI obstruction, stricture and fistula formation are absolute contraindications to capsule endoscopy. Intracorporeal pacemakers or other electronic medical devices are also not suitable for capsule endoscopy. Studies suggest that a history of abdominal surgery, endocrine disease (e.g., diabetes mellitus), neoplasia in the intestinal lumen, and Crohn’s disease of the small intestine are the main factors influencing the failure of capsule endoscopy to complete a total small bowel examination. When capsule endoscopy is chosen, the greatest concern of physicians is often the retention of the capsule. Wei Wei et al. reported a capsule retention rate of 10.0%, with Crohn’s disease patients having the most prominent problem, with a retention rate of 11.9%, followed by capsule retention in larger diverticula. Rondonotti et al. reported 183 cases of capsule retention in 700 examinations. Foreign countries reported 16.4% of those who could not complete the examination due to swallowing difficulties, and China reported a related incidence of 33.0%. Relevant examinations before capsule endoscopy, such as whole GI X-ray barium meal imaging, should be further standardized to exclude GI obstruction and large diverticula. Currently, the exploratory capsule system has been introduced, which will help to avoid the pain of surgery brought by capsule retention.